Three.js Loading manager Function

function addObject(objModel, mtlMaterial) {

    const manager = new THREE.LoadingManager();

    var viewerAlert = document.getElementById('viewerAlert');
    var viewerFiles = document.getElementById('viewerFiles');
    var countFiles  = document.getElementById('countFiles');

    manager.onStart = function(url, itemsLoaded, itemsTotal){
        function showViewerText(){
            viewerFiles.textContent = 'Started loading file: ' + url + '.\nLoaded.';
            countFiles.textContent = itemsLoaded + ' of ' + itemsTotal + ' files.';
        }

        if(viewerAlert.style.display == 'flex'){
            showViewerText();
        }
        else {
            viewerAlert.classList.remove('hidden', 'fade-out')
            viewerAlert.classList.add('fade-in');
            showViewerText();
        }
    };

    manager.onLoad = function(){
        viewerFiles.textContent = 'Loading complete!';
        countFiles.textContent = '';

        setTimeout(() => {
            viewerAlert.classList.add('fade-out');
            viewerAlert.addEventListener('transitionend', function(){
                viewerAlert.classList.replace('fade-out', 'hidden');
            });
        }, 800);
    };

    manager.onProgress = function(url, itemsLoaded, itemsTotal){
        viewerFiles.textContent = 'Loading file: ' + url + '.\nLoaded.';
        countFiles.textContent = itemsLoaded + ' of ' + itemsTotal + ' files.';
    };

    manager.onError = function(url){
        viewerFiles.textContent = 'There was an error loading ' + url;
        countFiles.textContent = '';
    };

    var mtlLoader = new THREE.MTLLoader(manager);
    mtlLoader.setPath('./assets/models/');
    mtlLoader.load(mtlMaterial, function(materials) {
        materials.preload();

        var objLoader = new THREE.OBJLoader(manager);
        objLoader.setMaterials(materials);
        objLoader.setPath('./assets/models/');
        objLoader.load(objModel,
            function(obj){
                obj.name = 'mainmodel';

                obj.position.set(0, 0, 0);
                obj.castShadow = true;
                obj.receiveShadow = true;

                // const box = new THREE.BoxHelper( obj, 0xffff00 );

                scene.add(obj);
                console.log(obj);
                // camera.position = obj.position;

                //scale all models to a relative scene size
                const model = new THREE.Box3().setFromObject(obj);

                let size = new THREE.Vector3();
                model.getSize(size);
                let scaleVec = new THREE.Vector3(1, 1, 1).divide(size);
                let scale = Math.min(scaleVec.x, Math.min(scaleVec.y, scaleVec.z));
                obj.scale.setScalar(scale * 2);
            },
            function(xhr){

            },
            function(error){
                console.error('An error occured!' + error);
            }
        );
    });
}